The present invention relates generally to communications systems and, in particular, to multi-rate speech coding systems.
Good quality voice services are in high demand, due in part to the emergence of global communication capabilities, such as those provided by cellular systems, satellite systems, landline systems, wireless systems, and combinations thereof. Digital speech coders typically used in these types of systems often operate at fixed rates that utilize a given amount of channel bandwidth. When enough channel bandwidth is available, fixed rate speech coders provide good quality voice services.
The transmission channel medium, however, is often capacity limited or causes excessively high bit error rates. When channel capacity changes, fixed rate coders are often unable to provide synthesized speech at a fixed delay, and they cannot dedicate additional forward error correction bits for protection against the noisy channel. In wireless applications, the channel capacity can change dramatically, and it thus imposes a variable limit on the maximum bit rate that can be passed through the channel.
Variable rate speech coders can reduce the coding rate when channel capacity diminishes, but the quality of speech can suffer. The quality suffers in part because of xe2x80x9cartifactsxe2x80x9d in the synthesized waveform at the boundary between coding rates. For example, when the variable rate speech coder changes from one coding rate to another, a user may experience a xe2x80x9cpopxe2x80x9d sound or a silent period due to a discontinuity in the synthesized speech waveform.
A significant need therefore exists for an improved method and apparatus for providing speech coding on a variable bandwidth channel.